MRI magnet systems typically include a cylindrical vacuum vessel having a bore for receiving the subject of the imaging. A cryostat or cryogenic containment vessel supported inside this vacuum vessel houses the coil of superconductive wire which provides the high strength, substantially homogeneous magnetic field necessary for imaging. The cryostat also contains liquid helium (which has a boiling point of about 4.2.degree. K.) for maintaining the wire below its critical temperature.
Copper power leads, extending from the leads of the coil to outside the vacuum vessel, are commonly used for energizing the coil from an outside power supply and for deenergizing the coil by connecting it to a resistive load outside the vacuum vessel. One of the problems with using permanently installed leads is that they are also excellent thermal conductors and thus transmit excess heat to the containment vessel resulting in increased consumption of expensive liquid helium.
To overcome the problem of continuous thermal conduction, power leads have been proposed which can be removed after energization of the coil. The use of removable leads usually requires the provision of an internal load resistor for protection of the magnet coil. Removal and reinsertion of the removable leads often results in the formation of ice which causes blockages thereby making the leads difficult to reinsert when the magnetic field must be changed or the coil discharged. Thus if the continuous heat leakage of the permanently installed leads were sufficiently low, their use would be preferable because of their greater reliability and greater safety because they can be continuously connected to the outside load resistance after charging of the coil.
For further information on the structure and operation of an MRI magnet system with fixed power leads, reference may be made to U.S. Pat. No. 4,655,045. An example of such a system with retractable leads is shown in U.S. Pat. No. 4,635,450. A lead assembly for a magnet coil which is in a tube which vents helium gas, is shown in commonly-assigned U.S. Pat. No. 4,625,193.